1. Field of the Invention
The present invention relates generally to radar techniques, and particularly to determining the angular direction of a radar target in a jamming environment.
2. Technical Background
The word “radar” is an acronym for radio detection and ranging that was coined by the U.S. Navy in the 1940s when the technology was in its infancy. Essentially, a radar system is one that alternately transmits radio frequency (RF) signals into a given search volume and then listens for reflections. By properly processing the radar returns, a radar system can determine the direction, range, altitude and/or speed of a target. Nowadays radar is used in a variety of ways. For example, the weatherman uses radar to track rain and snow storms. Police use radar to determine the speed of motorists. Civil aviation uses radar for air traffic control purposes. The military uses radar to track aircraft, ships, terrain and missiles. Needless to say, this is a small list of examples. The present invention, however, is directed toward determining the direction, or angular direction of a target relative to a radar, in the presence of jamming.
A monopulse radar system is type of radar that is often used for this task and can be formed using reflector antennas, phased arrays and etc. The term “monopulse” refers to the fact that the angle of arrival is estimated from data in a single pulse. The receive antenna may be divided into two segments such that the antenna receiver processes two signals in order to form two receive beams. The first beam is the sum beam (Σ) and the second is the difference beam (Δ). The amplitude of the sum beam (Σ) is symmetrical, with its maximum at the boresight. The amplitude of the difference beam (Δ) is antisymmetrical and is equal to zero at the boresight. In an array antenna, the angular direction of a target with respect to boresight (θ) is determined by computing a simple function of the “monopulse ratio,” Δ(θ)/Σ(θ), and comparing that value with a table of prerecorded such values versus angle of arrival. As those skilled in the art will appreciate, sometimes amplitude patterns of squinted beams are subtracted and added to form the Δ and Σ beams, and the simple function is simply the formed ratio; this type of monopulse is referred to as amplitude comparison monopulse. Sometimes the complex patterns of displaced phase center antenna beams are subtracted and added and the simple function is to extract the real part of the ratio. This second type of monopulse is referred to as full vector monopulse. This patent applies to both types of monopulse, but details below apply particularly to full vector monopulse. In practice, before the system is put in the field, the antenna is measured and calibrated such that the angular direction (θ) values of the monopulse table are accurate. This technique can also be employed in a planar phased array antenna to provide both azimuth angles (θaz) and elevation angles (θel). Briefly, the rectangular or planar phased array is divided into four quadrants on the receive side to derive a monopulse ratio for both the estimated azimuth angle (θaz) and the estimated elevation angle (θel).
One drawback to the above stated monopulse techniques for estimating the target angle (θ) relative to the antenna boresight is that they fail in the presence of main lobe jamming because the jamming noise renders the values in the look-up table substantially useless. The conventional approach to solving this jamming problem was introduced approximately 25 years ago by Applebaum and Wasiewicz, and provides a method for adaptively canceling a mainlobe jammer without suffering monopulse ratio distortion. In this approach, a linear array is divided into two large partially overlapped subarrays, with each subarray having an identical number of antenna elements. The conventional approach adaptively determines the individual weights ascribed to each to cancel the jamming signal. These weights are essentially the same for each subarray such that the adapted outputs are added to form a product sum Σ(θ) beam and subtracted to form a product difference Δ(θ) beam. The difference/sum ratio [Δ(θ)/Σ(θ)] is essentially independent of jammer cancellation because the jammer cancellation weights appear grouped in an identical factor of each subarray pattern and divide out when forming the ratio. One drawback associated with this approach is that the two partially overlapped subarrays are composed of different antenna elements, feedlines, and, if digital, A/D converters and receivers. The cancellation weights reflect these differences and they, in turn, limit the monopulse accuracy.
The approach described above has been extended by Applebaum and Wasiewicz to apply to rectangular aperture planar arrays with row-column weighting. As described above, rectangular planar arrays can be employed to provide azimuth difference/sum beams and elevation difference/sum beams for angle estimation purposes. (This method has been further extended to nonrectangular apertures by employing pattern synthesis to create product beams.) The extended approach creates azimuth product beams (made possible by row-column weighting or pattern synthesis) for cancellation and elevation product beams for azimuth angle estimation. The azimuth product beams are canceled out when the elevation angle estimation ratio is calculated. In order to obtain azimuth angle estimate, the process is reversed. One drawback to this approach is that it requires two sets of adaptive weights and controls, and in other words, two sets of adaptive beam forming hardware, or for digitized arrays, twice the digital processing, are required. More importantly, in all of these methods the receive aperture is shared by the jammer cancellation and angle estimation functions in some manner so that the performance of each are suboptimal. In particular, the aperture weight distribution appropriate for one function is not appropriate for the other, and so the aperture must be divided in some manner. In the planar array case, although the entire aperture is used in both functions, the post adaptive beams form line nulls that pass through the jammer angle, as opposed to the optimal point nulls.
Consider alternatively a straight-forward conventional approach that uses the entire aperture for canceling the jammer in the sum beam and the entire aperture for cancelling the jammer in the difference beam. Here, for planar arrays, point nulls are formed in the respective beams at the jammer angles. Adaptive nulling is applied to cancel the main lobe jamming in forming the sum beam and again in forming the difference beam by simply introducing sum and difference steering vectors in the weight computations for the respective beams.
FIG. 4 is a chart illustrating the post adaptive sum beam for the conventional approach. The top arc-shaped curve corresponds to a return that does not include jamming. The remaining three curves correspond to mainlobe jamming at three different angles. Adaptation to main lobe jamming has the effect of shifting the main lobe away from the angular direction of jammer. FIG. 5 is a chart illustrating the post adaptive difference beam for the conventional system. The difference beam substantially centered about 0° represents the “no main lobe jamming” case. When the conventional system adapts to main lobe jamming, it has the effect of shifting the difference pattern null to the angular direction of the jammer.
FIG. 6 is a chart illustrating the monopulse ratio for this conventional system. The system without jamming provides a clean antisymmetrical plot that passes through the origin (i.e., at zero amplitude, zero angle) as expected. In all three jamming cases, however, the angle estimation ratio deteriorates severely with jamming. The main reason for this is the loss of a null at the origin in the post adaptive difference beam (FIG. 5).
What is needed, therefore, is a system and method for monopulse angle estimation that addresses the drawbacks described above. A system is needed that applies adaptive jammer cancellation to the angle estimation process without distorting the sum and difference beams and without having to share the receive antenna aperture between the estimation and cancellation processes. Jammer cancellation is more effective if the entire aperture is applied optimally to cancelling the jammer. In this manner, an aperture weight distribution can be determined that is optimal for both suppressing jamming and maintaining sufficient target signal strength.